The present invention relates to an air-conditioning system that can be operated with CO2 for a vehicle. The system has a high-pressure section that is connected to the pressure output of a compressor and includes a gas cooler and a first branch of an internal heat exchanger, a low-pressure section that is connected to the suction inlet of the compressor and includes an evaporator, an accumulator and a second branch of the internal heat exchanger, and an expansion member connecting the high-pressure section to the low-pressure section, with optimum performance figures being assigned to optimum high pressures in the high-pressure section.
In air-conditioning systems and, of course, also in air-conditioning systems operated with CO2, the cooling performance to be supplied depends on the ambient temperature. The aim in practice is to operate with the most optimum performance figures possible, that is to say the ratio of the cooling performance to the drive power. It is known that the respectively optimum performance figures are in each case assigned to an optimum high pressure, that is to say are achieved at a high pressure that is optimum for the optimum performance figure. It has therefore become known to control the respectively optimum high pressures by means of a controllable expansion valve. The aim here is to move to the optimum high pressures for the respectively optimum performance figures as accurately as possible. This requires a high cost outlay in relation to the adjustable expansion valve, in relation to the high-pressure sensors needed and in relation to the design of the control system overall.
The increased complexity of air-conditioning systems operated with CO2 for the additional internal heat exchanger and for the high-pressure regulation by means of an expansion valve, which are not necessary in the case of air-conditioning systems operated with other refrigerants, leads to the air-conditioning systems operated with CO2 being made considerably more expensive. Although CO2 as a refrigerant is significantly more environmentally friendly than other refrigerants and also offers other advantages, and although the performance of air-conditioning systems operated with CO2 is at least equivalent, even if not superior to, the performance of air-conditioning systems operated with other refrigerants, the higher price places practical use at risk.
The present invention is based on the object of providing an air-conditioning system that can be operated with CO2 of the type described above which permits a simplified construction and therefore a price reduction.
This object is achieved by the individual components being designed and/or matched to one another in such a way that if the high pressures deviate by up to xc2x130% from the optimum high pressures, the associated optimum performance figures are reduced by no more than 20%.
Trials have shown that it is possible to design an air-conditioning system in this way, so that a satisfactory cooling performance with a satisfactory performance figure is achieved, without complicated high-pressure regulation being necessary. It is then possible to provide significantly simplified regulation of the high pressure which, for example, operates with larger graduations. Under certain circumstances, it is even possible to manage with a single fixed throttle, that is to say to dispense completely with regulation of the high pressure.